In the method of using electrophotography printing and electrostatic development to obtain an image, the electrostatic image formed on a photosensitive body is developed by a toner given a charge by rubbing in advance, then is fixed. The fixing methods include the heat roller system of fixing the toner image obtained by development by a pressing and heat roller and the noncontact fixing system of fixing the image using an electric oven or flash beam of light. To get through these processes without problem, the toner first of all has to have a stable electrostatic charge. Next, the fixability to paper must be good. Further, since the apparatus has a fixing unit which is a heater and the temperature inside the apparatus rises, the toner must not form blocks. Recently, energy saving has become essential and the fixing unit in the heat roller system has been made lower in temperature. Therefore, there are strong calls for the toner to feature the ability to fix images to paper at lower temperatures, that is, a low temperature fixability. Further, recently, along with the spread of full color electrophotography systems, a toner enabling the formation of a glossy image has been sought.
The toner binding resin has a large effect on such toner characteristics. A polystyrene resin, styrene-acryl resin, polyester resin, epoxy resin, polyamide resin, etc. are known. Recently, however, due to the ease of obtaining a good balance of transparency and fixability, the excellent transparency, and the properties suitable for a full color toner, polyester resins have been looked at with particular interest.
However, to obtain a good fixability for a toner using a polyester-based resin as a binding resin, as described in Japanese Unexamined Patent Publication (Kokai) No. 4-12367 and Japanese Unexamined Patent Publication (Kokai) No. 59-128558, improvement of the fixability by a monomer has been proposed, but the obtained resin has the problems of a high molecular weight and the inability to obtain the targeted fixability.
Further, as shown in Japanese Unexamined Patent Publication (Kokai) No. 7-140714, Japanese Unexamined Patent Publication (Kokai) No. 2002-287427, Japanese Unexamined Patent Publication (Kokai) No. 2002-202634, and Japanese Unexamined Patent Publication (Kokai) No. 4-313760, it has been proposed to blend polyester resins with different softening temperatures and molecular weights, but with this method, adjustment of the balance between the fixability and the offset resistance is difficult. Further, there is the problem of the difficulty in adjusting the balance with the gloss between the polymer component or bisphenol A derivative component or unsaturated fatty acid or other monomer component.
Further, in Japanese Unexamined Patent Publication (Kokai) No. 4-362956, Japanese Unexamined Patent Publication (Kokai) No. 8-320593, attempts are made to mix into a nonlinear polyester a low melting point linear polyester to improve the low temperature fixability. Further, Japanese Unexamined Patent Publication (Kokai) No. 10-339969, Japanese Unexamined Patent Publication (Kokai) No. 2000-305316, etc. study use of linear polyesters.
Further, Japanese Unexamined Patent Publication (Kokai) No. 8-30027 proposes a resin given a good low temperature fixability, but this has the problem of an insufficient balance with the grindability and insufficient durability for long term printing resistance.
Thanks to these technologies etc., toner resins improved in fixing performance have been developed, but the market demands on fixing performance have become further severe. The above-mentioned technology did not go so far as to develop a resin maintaining the blocking resistance, offset resistance, and other required properties and yet having low temperature fixability and gloss able to satisfy market demands.
Further, recently, energy saving has become essential. In the heat roller system, the fixing units have been made lower in temperature. Therefore, performance enabling images to be fixed to paper at lower temperatures, that is, low temperature fixability, is now being strongly sought from toners. Further, a broader working range is being required. Therefore, there have been demands for a greater range of fixing temperature of toners.
For example, Japanese Unexamined Patent Publication (Kokai) No. 4-362956 studies a binding resin making use of both a nonlinear polyester and a linear polyester. Specifically, it discloses a toner having a range of fixing temperature of 50° C. or more making use of both a nonlinear polyester having a softening temperature of 110 to 116° C. and a linear polyester having a softening point of 85 to 102° C. However, the toner described in Japanese Unexamined Patent Publication (Kokai) No. 4-362956 has a wide range of fixing temperature, but a high minimum fixing temperature of 150° C. or more and therefore still an insufficient low temperature fixability.
Japanese Unexamined Patent Publication (Kokai) No. 4-313760 studies a binding resin using two types of linear polyester with different softening points. Specifically, it discloses a toner having a range of fixing temperature of 30° C. or more using a high softening point linear polyester with a softening point of 112 to 123° C. and a low softening point linear polyester with a softening point of 89 to 92° C. However, the toner described in Japanese Unexamined Patent Publication (Kokai) No. 4-313760 has a minimum fixing temperature of a high 150° C. or more and is still insufficient in low temperature fixability.
Therefore, in view of the recently increased speeds, smaller sizes, and greater energy savings of copiers, a further lower temperature fixability and wider range of fixing temperature are desired.